LCOS display device is a latest liquid crystal display device. Unlike a conventional liquid crystal display device, LCOS process is compatible with CMOS process, thus driving circuit can be implemented on silicon wafer. The LCOS display device may have a small size and high resolution since the CMOS process can be utilized to fabricate active pixel matrix on silicon substrate.
Typically, an ideal LCOS is flat, smooth and of high reflectivity, so as to ensure an appropriate arrangement of liquid crystals, a uniform thickness of the liquid crystal layer, and no distortion of light. Thus, high quality mirrors are required to control the reflected light accurately. It is a key factor for advanced applications, such as projection television, etc.
An existing method of fabricating mirrors for LCOS display device is disclosed in, e.g., U.S. Pat. No. 6,437,839, which includes the following steps. As shown in FIG. 1A, a dielectric layer 101 is formed over a silicon substrate, a metal layer 102 of an Al—Cu alloy (a content of Cu is 0.5%) is formed over the dielectric layer 101 by sputtering method; an anti-reflective layer 103 is coated over the metal layer 102 to protect the metal layer 102 during exposure; and a photoresist layer 104 is formed over the anti-reflective layer 103 and is subjected to exposure and development, thereby forming an opening pattern 107.
Referring to FIG. 1B, the anti-reflective layer 103 and the metal layer 102 are etched using the photoresist layer 104 as mask until the dielectric layer 101 is exposed, so as to form a trench 105.
Referring to FIG. 1C, an ashing process is performed to the photoresist layer 104 and the anti-reflective layer 103; the residual photoresist layer 104 and the anti-reflective layer 103 are further removed by using basic solution; and an insulation layer 106 is formed over the metal layer 102 by HDPCVD (High-Density Plasma Chemistry Vapor Deposition) method, so that the trench 105 is filled with the insulation layer 106.
Referring to FIG. 1D, the insulation layer 106 is planarized through CMP (Chemical Mechanical Polishing) method, and then is dry-etched until the metal layer 102 is exposed, thus forming mirror array 108.
FIG. 2 is a schematic diagram of mirrors with pits for LCOS display device fabricated by the existing method. During removing the residual photoresist layer 104 and the anti-reflective layer 103 with basic solution, the Al—Cu alloy in the metal layer has an electrochemical reaction with the basic solution, and thus Al is consumed, so that the metal layer has many pits, and further the mirror formed subsequently has many pits. When mirrors are observed in an optical microscope with an amplification rate of 500, many white bright spots can be found, i.e. the pits with a size ranging from 1 μmm to 3 μm.
In the prior art, during fabrication of mirrors for LCOS display device, particularly when removing the residual photoresist layer and anti-reflective layer with basic solution, the Al—Cu alloy in the metal layer has an electrochemical reaction with the basic solution, and thus Al is consumed, so that the metal layer has many pits, and further mirrors formed subsequently have many pits. Consequently, mirrors may be degraded.